Scheduling: Examining the Start to Finish Relationship

From time to time I receive Emails that raise intriguing issues of general interest. Here is such a question (edited for web page clarity.)

Norris S. Goff wrote 1/14/09,

"Subject: Re: PMBOK® Guide 4th edition

Max, please give me your view on the following that I recently reported to the Project Management Institute. [In The Guide to the Project Management Body of Knowledge,] 4th Edition [2008], page 138,Start-to-finish states:[1] "The completion of the successor activity depends upon the initiation of the predecessor activity."

I believe that statement should read: "The completion of the predecessor activity depends upon the initiation of the successor." A most common example is that of a relay race. Racer #1 (the predecessor) cannot complete until he has passed the baton to racer #2 (the successor).

The response I received from John Zlockie [PMI]: "I believe that the current descriptions of PDM dependencies in the PMBOK® Guide - Fourth Edition are accurate."

I think the relationships should read more logically according to the 1996 PMBOK® Guide, where the four possible types of logical relationships were expressed as follows:

Finish-to-start: the 'from' activity must finish before the 'to' activity can start.

Finish-to-finish: the 'from' activity must finish before the 'to' activity can finish.

Start-to-start: the 'from' activity must start before the 'to' activity can start.

Start-to-finish: the 'from' activity must start before the 'to' activity can finish.

I agree that the 4th Edition doesn't seem to make sense, but it is the same as the 2004 (3rd) Edition (page 132) and 2000 Edition (page 69). If the successor activity is nearing completion and the 'predecessor' activity hasn't even started, then it can hardly be a 'predecessor' activity! I think your wording would be consistent with the 1996 version. Not bad - only took nine years for someone to spot a serious flaw.

"Well, Max and Norris, I enjoy items of scheduling theory even those without much practical use!

The relay handover involves a finish-to-start dependency with a negative lag. That is so because the latter activity is the 'dependent activity'. I think it is artificial to say the finish of the runner #1 has to wait for the start of runner #2. There is a handover for which the timing is determined primarily by the arrival of #1 and secondarily by the skill of handover (unless it's botched).

An S-F relationship can arise when the finish timing of an Activity A is dependent on an independent start timing of an Activity B (which will likely finish later than A). The term 'successor activity' does not describe the logic well, although the applicable start and finish have a successor sequence in both logic and timing. 'Dependencies' (and activities/milestones) are what a CPM schedule sets out to model.

(PMI has fallen into a terminology trap of over-standardization, in which a 'predecessor' is in a logic relationship that doesn't have to mean timing sequence, which is counter-intuitive!)

Example: 'A' is working at a shift-based job and must have an overlap to give a report to replacement 'B', who is held up in a snowstorm. The actual start of the B's shift controls the finish of A's - a start-to-finish dependency. This is not equivalent to a relay race because activity A Shift that started sooner has the dependent finish, which cannot be until the independent next activity, B Shift, has started, i.e. an S-F logic sequence with a few minutes of positive lag.

Right?

However, I don't recall ever using an S-F link in a real or hypothetical schedule! It could be avoided in the above [snow storm] example by an F-F dependency between substitute B's skidding and digging to work and 'A' leaving work.

To be silly, I could zoom in on the relay race handover and find a sequence of F-S dependencies: #1 nears the transfer box; #2 (already in the box) starts running; #1 gets to two-arms length of #2; #1 places baton in hand of #2; #2 takes hold of baton; #1 releases baton and is finished (in perhaps more ways than one! And so am I)."